1) Field of the Invention
The present invention relates to a mirror device and an optical apparatus suitable for use in an optical communications system.
2) Description of the Related Art
As is widely well known in these days, to convert networks with Wavelength Division Multiplexing (WDM) as the core thereof into optical networks, has been progressed in high speed, in order to accommodate outstandingly increasing internet traffic. Although WDM of these days mainly has a point-to-point network form, it is expected that it will advance to ring-shaped networks and mesh-formed networks in the near future.
To support this, each of the nodes forming a network will be capable of performing processing such as add/drop of an arbitrary wavelength, thereby making it possible to perform processing of whole Optical Cross Connect (OXC) without electric conversion so that dynamic setting/releasing of paths will be performed on the basis of wavelength information. As to such progress in photonic networks which make the best use of optical technology, a detailed discrimination is described in, for example, the journal of the Institute of Electronics, Information, and Communication Engineers, February 2002, pp. 94 through 103.
FIG. 15(a) and FIG. 15(b) each show conceptual diagrams of an optical apparatus 100 (hereinafter, referred to as a wavelength selection switch 100) which operates as a common wavelength selection switch. FIG. 15(a) is a schematic upper view of the wavelength selection switch 100; FIG. 15(b) is a schematic side view of the wavelength selection switch 100. The wavelength selection switch 100 shown in FIG. 15(a) and FIG. 15(b) as an example, has a function of distributing a wavelength input from a single input port 101 to an arbitrary output port 102-1 through 102-4, and for example, it is installed in nodes in ring-shaped networks and mesh-shaped networks.
Here, the wavelength selection switch 100 includes: an input/output optical system having a single input port 101 and multiple output ports 102; a spectral element 120; an light-gathering optical system 130; and a mirror device 140 having multiple movable reflectors 141 arranged in a spectroscopic direction as shown in FIG. 15(a). Then, after light at multiple wavelengths input from the input port 101 is demultiplexed by the spectral element 120, demultiplexed light is concentrated to removable reflectors 141 each corresponding to one of the demultiplexed wavelengths. The reflected light is switched to an arbitrary output port 102 by means of changing the angles of the movable reflectors 141.
The input/output optical system 110 includes: a single first port (input port) 101; and multiple (here, four) second ports (output ports) 102-1 through 102-4 with a first port 101 at the end thereof within the same plane. In this instance, each of the ports 101 and 102-1 through 102-4 can be constructed so as to have a collimate means, such as a lens, for optically coupling an optical fiber individually arranged with the spectroscopic element 120.
In addition, the spectroscopic element 120 demultiplexes light input from the input optical system forming the input/output optical system 110, and it can be provided as a form of diffraction grating. Further, the light-gathering optical system 130 outputs each light component, which was demultiplexed by the spectroscopic element 120, to the movable reflectors 141 of the mirror device 140 arranged thereafter as collimated light, and it can be provided as, for example, a light-gathering lens of a permeable type or a reflection type. In this instance, in the drawing, the example shows a construction which employs a light-gathering lens of a permeable type.
Further, the mirror device 140 includes multiple movable reflectors 141 arranged in accordance with the demultiplexing direction in the spectroscopic element 120. Each of the movable reflectors 141 reflect one of the light components which is made into collimate light by the light-gathering optical system 130, and is also capable of deflecting the reflection light of each light component. Then, the light reflected by these movable reflectors 141 pass through the light-gathering optical system 130 and the spectroscopic element 120 once again, and is then introduced to the input/output optical system 110.
As a result, the wavelength selection switch 100 of FIG. 15(a) and FIG. 15(b) is constructed so as to realize mutual and bi-directional optical coupling between the first port 101 and the multiple second ports 102-1 through 102-4, which form the input/output optical system 110, through a reciprocate optical path by way of the spectroscopic element 120, the light-gathering optical system 130, and the mirror device 140, by means of setting the reflection surface angle of the above described movable reflectors 141, so that the wavelength selection switch 100 is adapted to operate as a wavelength selection switch in which paths to be optically coupled can be set by the unit of wavelength for each of the port 101, and 102-1 through 102-4, which form the input/output optical system 110.
For example, in a case where the first port 101 functions as an input port and the second ports 102-1 through 102-4 function as output ports, it is possible to selectively introduce light from the first port 101 to one of the second ports 102-1 through 102-4 through the above described reciprocate path by the unit of wavelength. In a case where the second ports 102-1 through 102-4 function as an input optical system and the first port 101 functions as an output optical system, it is possible to introduce light from the second ports 102-1 through 102-4 to the first port 101 through the above described reciprocal optical path by the unit of wavelength.
The arrangement of the movable reflectors 141 forming the mirror device 140 is generally realized by a mirror system 142 forming an MEMS (Micro Electro Mechanical Systems) mirror array. Movable mirrors as the movable reflectors 141 are arranged corresponding to wavelengths demultiplexed by the spectroscopic element 120. The movable reflectors 141 are constructed in such a manner that the tilt angles thereof are changeable. As shown in FIG. 15, it is possible to determine output ports for wavelength components in accordance with the tilt angles of the movable reflectors 141.
Further, the mirror system 142 is hermetically closed with the sapphire glass 143 in order to prohibit effects on the reflection surface of the movable reflectors 141 from change in humidity and mixture of foreign matter. That is, light passes through the sapphire glass 143 as a permeable window, thereby light being input/output to the movable reflectors 141 forming the mirror system 142.
In this instance, the sapphire glass 143 as a permeable window is a material selected from the viewpoints of mechanical strength and optical permeability. In addition, if multiple reflection occurs between the sapphire glass 143 and the movable reflectors 141, as shown in FIG. 16, it becomes crosstalk to the output ports 102. To prevent this, as shown in FIG. 15(b) and FIG. 17, for example, the sapphire glass 143 is tilted with respect to the arrangement direction of the input/output ports 101 and 102, thereby preventing the mixing of reflection light to output ports 102 other than a destination output port 102 to which light is to be introduced.
In the thus-constructed wavelength selection switch 100, which is a wavelength selection switch having the first port 101 as an input optical system and the second ports 102-1 through 102-4 as an output optical system, it is possible to switch output ports 102 which are to be output to destinations of wavelengths input from the first port 101 by means of setting the tilt angle of the mirror system 142.
Here, since the wavelength selection switch 100 as described above includes multiple elements which generate Polarization Dependent Loss (PDL) as represented by diffraction grating forming the spectroscopic element 120, it is difficult to suppress PDL in the whole of the wavelength section switch down to not larger than system specification value only by means of restraining PDL by the unit of element. Thus, as shown in FIG. 15(a) and FIG. 15(b), for example, a λ/4 wave plate 150 is arranged before the mirror device 140, thereby canceling PDL.
However, the wavelength selection switch 100 as described above is incapable of resolving a phase difference between deflections (between ordinary light and abnormal light) generated at the time light passes through the sapphire glass 143 having birefringence even though the wavelength selection switch 100 includes the λ/4 wave plate 150. In particular, the angles of light passing through the sapphire glass 143, which forms a permeable window, with respect to the sapphire glass 143 are different between light input from the first port 101 to the movable reflectors 141 and light reflected by the movable reflectors 141 to be introduced to the output ports 102, so that effects of birefringence are different between input light and output light.
Further, such effects of birefringence differ depending upon the degree of tilt of the sapphire glass 143 which is for restraining the above described cross talk. This is because the angles of input light and output light with respect to a crystal axis forming the sapphire glass 143 depend upon the tilt angle of the sapphire glass 143.
The following patent document 1 and patent document 2 describe constructions for removing effects of birefringence by means of making the angle of an input beam to the sapphire crystal in the crystal identical to the axis C of the sapphire crystal for the purpose of preventing deterioration of the efficiency of coupling to the sapphire crystal due to birefringence in a case where the sapphire crystal is similarly a permeable window in a laser module.
Further, the following patent 3 describes a construction for compensating for polarization mode dispersion by means of controlling the temperature of the crystal by providing a construction which makes the beam angle input to a birefringence crystal, such as potassium niobate, identical to the a axis direction, and makes the diffraction direction identical to the axis C and the axis b directions.
[Patent Document 1] Japanese Patent Application Laid-open No. 2005-136119
[Patent Document 2] Japanese Patent Application Laid-open No. HEI 8-148594
[Patent Document 3] Japanese Patent Application Laid-open No. 2003-43418
However, the above patent documents 1 through 3 propose effects of birefringence onto only a single beam, and they do not propose a construction for suppressing PDL on the assumption of multiple modes of angles with respect to the crystal axis of the sapphire glass at the time input light and output light to movable mirrors pass through the sapphire glass which forms a permeable window.
For example, the above described wavelength switch has a feature such that a beam input from a single input port 101 is coupled to one arbitrary port, out of the multiple output ports 102, or that a beam from an arbitrary one of the beams from multiple input ports is coupled to one output port by means of changing the angle of the removable mirror, so that consideration must be paid to effects of birefringence of multiple beams at the same time.
More specifically, as shown in FIG. 18, a construction is assumed which has a single input port (port #0) 101 and four output ports (port #1 through port #4) 102. At that time, following the technology described in the patent document 1 and the patent document 2, it is considerable that the optical axis AX0 which is made by light from the port #0 within the crystal is made to be identical to the direction AXC of the axis C which is the crystal axis of the sapphire glass 143, in order to suppress PDL of light from the port #0, as shown in FIG. 18.
However, PDL of a beam output from each port #1 through #4 depends upon the sum of phase differences which are given to a beam input from the port #0 and beams from output ports #1 through #4 by birefringence within the sapphire glass 143. Accordingly, as shown in this FIG. 18, even if the light axis AX0 formed by the light from the port #0 is made to be identical to the axis C direction AXC, which is the crystal axis of the sapphire glass 143, in order to suppress PDL of the light from the port #0, beams respectively returning to output ports #1 through #4 after being reflected by the movable mirror 142 always deviate from the axis C direction AXC of the sapphire glass 143, so that PDL is resultantly increased.
As an example, when PDL is calculated from a phase difference due to birefringence by use of Jones vector, in a case where PDL of 1 dB initially exists in a diffraction grating or the like, for example, the thickness of the sapphire glass 143, as a permeable window, of 1 mm and the angle difference of 10° within the sapphire glass 143 made between the port #0 and the port #4 makes a 0.71 dB of PDL remained even if the λ/4 wave plate 150 is arranged. Although it is desirable that PDL should be restrained down to approximately 0.4 dB as a matter of system design of the wavelength selection switch, such a construction significantly exceeds this value.